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A&. Space Res. Vol. 25, No. 2, pp. 315-322, 2000

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APSIS - AEROGEL POSITION-SENSITIVE IMPACT SENSOR: CAPABILITIES FOR IN-SITU COLLECTION AND SAMPLE RETURN

J.A.M.McDonnell’, M.J. Burchell’, SF. Green’, M. Leese’, D. Wallis’, J.C. Zamecki’, D.J. Catling’, D.E. Brownlee*, P. Tsou’, L. Colangeli4, E. Bussolett?, G. Drolshagen6, C.R. Maag’ and H. Yano’.

‘Unit for Space Sciences and Astrophysics, University of Kent at Canterbury, UK * Dept. of Astronomy, University of Washington, USA 3NASA Jet Propulsion Laboratory, USA 40sservatorio Astronomic0 di Capodimonte, Italy ‘Istituto di Fisica Sperimentale, Istituto Universiatrio Navale, Italy %pace Environment and Effects Analysis Section, ESTEC, The Netherlands ‘T & M Engineering, Florida, USA ‘Earth Science and Solar System.Exploration Division, NASA Johnson Space Center, Houston, USA

ABSTRACT

The capabilities of combining Aerogel (A) as a passive target and position-sensitive impact sensing (PSIS) are examined; experimental performance factors are established from tests on Hypervelocity Impact Facilities at the University of Kent and previous space deployment. Results of system requirements analysis favour the deployment of large area detectors. Analysis of the in-flight impact sensor signals yields the position, time and exposure pointing direction of the tracks containing the impactor residues, which are analysed after retrieval.

01999 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION

Real time detection in space and retrieved sample analysis of meteoroids and space debris both offer valuable opportunities for space environmental and astronomical studies. Information from the two approaches is complementary and yet, because the overlap is limited in space opportunities to date, the benefits of potential cross-fertilisation in the two data sets are often missed. Hitherto the two techniques have not been combined in a flight detector of significant exposure in the space environment.

Real-time detection often utilises the impact energy to trigger a measurement and, consequently, destroys the physical entity of the particle. Retrieved samples or systems often attempt to preserve the particulate’s morphology or chemistry but are often serendipitous in seizing flight opportunities and, even where a dedicated experiment is deployed, information on the particle trajectory is not usually measured in an inertial frame. APSIS addresses the merging of real time detection and retrieved sample analysis.

By measurement of the position and time of the impact on a retrievable collector (Figures la, b), post flight scanning and residual analysis can not only provide chemical and physical data on the impactor but further, if combined with the velocity vector of the spacecraft at the instant of detection, can yield the trajectory in the appropriate heliocentric or geocentric reference frame (Yano, 1995). The source of the impactor residues can thus be sought.

315

Fig. I(a)* Aerogef Position Sensitive rmpact System S~hern~~~ showing typicaf dep~oy~nt on Eureca type spacecraft; the impact detection requires, ideally, sampling of the time, altitude and attitude for the ex~e~ment. The fomer - time - is suf~cj~nt if the orbital data and spacecraft attitude are well ~ha~~~~erised.

APSIS CONCEPTS

Refer - Residues

The encounter v&city of ~~j~u~ates with any spacecraft in few Earth orbit is ~~~a~~~ some 9 km s-t for space debris and over 20 km s-r for meteoroids. Impact pressures between solids of ‘%ommanpiace density” (e.g. > Ig cm‘3) readily melt impactors at impact speeds of some 3 to 4 km s-‘.and potentially vapourise material above some 5 km s-i. hollowing suggestions (Drtrry, 1986), the first author i~vest~~ted~ in BSA’s study of a ~tenti~~ Cometary Atmosphere Sample and Return mission (CAESAR 1986, Eberhardt et al 1986), the use of aerogel as a capture medium. First tests were performed at the Ernst Mach Nyperveiocity Impact Faciiity in 1986 and also on “~~d~e~~” foams. Mission concepts and retrieval techniques using aerogel are now ~rnb~i~ into the NASA Stardust Missian to visit a comet arid collect ~nt~~~et~ and interste~~~ dust.

SueeessfuX retrieval of space ~a~~~ulates in ~&entr~c orbit in aerogel has been ~~o~~ by a ‘~haud~~ of authors, e.g.Tsou <I993), Tsou et al (1993, I99S), BrownIee eb al (1988, 1994), BurcheIt et al. (1996, 2999) Laurance (1990). A~~o~gh it has not yet demonstmted the intact captum of ~~~~~~~ ~a~~u~ates, this fact may indeed reflect the very successful, yet selective, intact capture of space debris at the iower impact velocity of 9 km s’t, This veiocity contrasts with ~teoro~ds at some 20 km s** and these are likely tu be va~our~s~. St~dust~s interpretation of the fow flux of inters&&r dust in inte~l~et~ space will. demand extreme patience in terms of

Fig. l(b). Retreival parameters. The track predicted (at x, y) which identifies optinmm search area; measurement yields particle track length and residue - estimate of particle mass and velocity. Actual track angIes are measured in the aerogel and, from the ins~ment attitude at &,, the micrometeoroid orbit can be reconstructed; the chemistry of the particle is also determined,

3t7

Bending I Surface wave r

Fig. 2. P~sitiou sensitive system using a bending wave, more approp~ate for ;x thin large area detector. Four sensors yield redundancy plus improved sensitivity and accuracy.

scanning for impact sites and, perhaps, ingenuity in terms of inte~re~tion of data, At the cometary encounter velocity of 6.1 km s-r, however, the capabilities of aerogel for intact capture are w&1 demonstrated for the cometary encounter phase.

Heritage of ~o~n~~~i~ Available

Parameters such as the detector attitude, velocity vector and spatial position will change with time; time, itself, is a most critical parameter in the i~te~~tation of an impact site and vita! to the derivation of orbital elements and position of the detector relative to a meteoroid stream or cometary plane. The instant of detection must be known therefore and, because a detector is likely to acquire more than one impact, we must know where each timed impact site is located. Figures 2 and 3 show two Gon~~mt~o~ for ~s~~~ sensitive detection in aerogei. The d~t~~on of an event must trigger the acqn~s~~on of the space~r~t attitude and register a clock status, later to be folded with the spacecraft orbital parameter file. We note that the rno~n~ of a particle can be detected ~M~~nne~l 1969, McDonnell et al ( 198 11, with high sensitivity and that this is largely insensitive to the nature of the target if it is capable of transferring the high frequencies of a momentum exchange.

Results from the detection time yield the ~~e~~~~~~ pointing direction and position - ~ywhe~ in the Solar System for a tracked mission Post flight analysis may, further, yield an incident direction relative to the

Fig. 3: Position sensitive detection system for aerogef bIock, shown using the lon~i~dinal compression wave in aerogel, plus four sensors. Impedance matching to the PZT crystals is performed on each of the four comers.

PZX impedance Transformation

Fig. 4. Impedance mat~hiug by the use of density graded aerogei yields high sensitivity momentum detection from aerogeI.

@~~e~6~ent ,- eg. a track in aerogel. This can be folded with the flight exposure to yield the true inertial vector direction of the particle at collection relative to the (moviug} detector, e.g. Buroheil et CZI. (1999). Lastly, the particle velocity may possibly be deduced from the track. If this is possible, then vector addition of this ~ack~e~ved relative velocity and the spacecraft velocity vector will yield the full particle trajectory p~ameters. Quite often the relative impact velocity is low comp~ed with the orbital velocity and, especially, the hei~ocentr~c spacecraft velocity; hence potential errors in the track-derived velocity are reduced in terms of significance to the inertial vector.

~~l~-~~act Acc~mnlatio~ with Timeli~e.

The detector needs to record the impact time and spatial position at collection; an additional factor of vahxe is the ~pli~de (eg. the momen~m induced in the target) which can assist in dis~~~na~on and could yield ad~tion~ impact factors. The number of events distin~ishable is given simply by the inverse square of the (fractional positional disc~~nation in linear Dimensions, e.g. 100 particles per detector if the fractional x and y errors are each .lO% on a ~t~~l~ array. To achieve good pos~tioual accuracy, a high signal to noise ratio is required; for this reason im ante matching to the sensors is considered vital (Figure 4). Figure 5 shows a typicaf signal set from 4 sensors and the potential reconstmction of the impact exposure history in terms of particle momenta (amplitudes) and positional determinations (gaussian error profiles).

Aerogel Tracks.

The use of aerogel as a capture medium has been investigated in a number of laboratories. Following earlier expe~ments in Europe, the first ~ubiisbed report of capture appears to be by Tsou et al. (1988), where capture at 5 km s-t of 25 - 200 micron sized grains in aerogel of density 150 m”3 was reported. The figures in that paper show the now classic ‘&u-rot” shaped tracks in the near ~~sp~e~t aerogel. Tsou et ai. (1990) used ~Ie~trostat~c ac~leration of sub~~cron sized iron grams to 10 - 12 km s-r, and observed captured particles; in the same paper they also report a lack of captured particles at the end of tracks for impacts at 14 km s‘r. There is thus the suggestion that at higher velocities the degree of intact capture decreases. Barrett et al. (1992) reported on capture experiments for 100 micron sized grains at 5 - 7 km se”, but using aerogels of differing densities (20 - 120 kg m se3>.

Ferhaps the most detailed reports of track len s and capture size are by ~urcheil and Thomson (1996) and Burchell et al. (1999). They report on capture at 5 km S-I of soda-glass, olivine and iron grains of size 75 to 355 herons in aerogel of density 93 kg m‘“. For example, they found that for olivine grains the track length (T microns) observed in the aerogei could be given as a function of grain diameter (d microns):

T = 920 -c 48.8d (units of microns).

They also found that the typical capered diameter D could si~larly be given by:

D = 32 c 0.42d. (units of microns). (2)

APSIS - Instrument Impact 319

Imnact nositions nredicted

x, y =f&b, A27 A37 &)

x, y =fdT2-Tl,TvTI, Ta-Td

s4

Fig. 5. Typical signals derived for a position sensitive impact system indicating how the x-y co-ordinate can be derived from either the amplitudes (Al - &) or the relative timing (TI - TJ). Potential reconstruction of the incident momentum and position for a set of 4 impacts accumulated on aerogel is shown as gaussian profiles.

There is probably insufficient data to reliably extrapolate these relations to other impact velocities or aerogel densities and thus care should be taken when using aerogels of densities far from 93 kg mm3. By carrying out impacts at oblique angles, Burchell et al. (1999) demonstrated that the direction of an impact could be found to f 2”. This is of importance in any application hoping to derive the pre-impact direction of flight for a particle in

space.

A recent NASA report (H&z et al 1998) confirms many of these findings. In particular it suggests that for velocities of 5 and 6 km se1 and low density aerogels (10 - 20 kg rn-“) faster particles travel noticably shorter distances in the aerogel. This implies a complicated dependence of track length on velocity.

The acoustic and shock properties of aerogels have also been the object of study. Holmes and See (1992) and Grover et al. (1992) report on measurements of the shock speed in aerogel (density 128 kg m3) at impact speeds of a few km s-‘, and attempt to derive an equation of state for the aerogel. Extrapolation of this to other aerogel densities remains an open issue. Gibiat et al. (1995) report on acoustic propagation in aerogels, including information on attenuation, boundary effects etc. They found that neither the classical theory of propagation in a homogeneous media nor the Biot theory for porous materials apply. Experimental investigations for particular set- ups for specific applications are thus required.

Real Time Detection.

Positional discrimination of impacts at 68 km s-r for cometary particles was convincingly demonstrated on the Giotto mission (e.g. McDonnell and Abellanas 1972, Grtin and McDonnell 1979) where the dust shield was instrumented with 3 piezo electric (PZT) detectors on the aluminium shield. The use of relative amplitude or relative timing is a unique identifier of the position using 3 measurements, if impact occurs within that “triangle”; with 4 sensors the measurement is unique and the redundancy of measurement also increases the reliability. By combining both amplitude and time, we can enhance the positional discrimination further.

Detection on Giotto of impactors on the dust shield was via the bending wave induced in the thin (meteoroid bumper) 1 mm plate; the propagation velocity is some 2 km sW1 and, though all frequency components are initially in phase, propagation is dispersive; longitudinally resonant PZT detectors pick out a single frequency well above spacecraft ambient noise (usually 100 kHz or 200 kHz). A selective bandwidth and peak detection extracts the

320 J. A. M. McDonnell et al

peak envelope of that frequency. Principles have been incorporated in proposals for the Rosetta Dust Flux Analyser (McDonnell et aE 1994, 1996), now assimilated within the Ralian GIADA (Grain Impact Analyser and Dust Accumulator) detector (Lamy 1998). The essential ~asurement outcome of a PZT peak m~sure~nt is a value related linearly with the ~o~e~t~~ exdmge during initial impact (McDonnell 1969). With aerogel, the momen~m will almost exactly match the particle momentum; but, because of the thicker target dimensions and configuration for aerogel capture, we may not always be able to detect the bending wave. An alternative, the longitudinal compression wave, is being studied with special reference to the acoustic transmission properties of aerogel; critical to detection is the impedance matching from the aerogei capture medium density (typically 10 kg m‘” or less) to the PZT detector (density 3000 kg m-s). For this reason the use of a graded density of aerogel is being studied (Tsou, personnal co~unication 1997) to couple more efficiently. Altematives, such as a hollow cone, could achieve this transfo~ation by wave guiding. The mashing concepts of graded aerogel are shown in Figure 4.

System Requirements.

System r~uirements are very mission specific in practice and also depend on the launch and retrieval conditions; convincing demonstration of the deployment of aerogel, however, has been achieved within the Space ShuttIe orbiter environment where acoustic noise is a significant environments hazard. Exposure in space presents no problems to aerogel but, if PZT crystals are deployed, temperature control to limit maxima to some 200°C may be needed to avoid exceeding the Curie point. Insolation control and radiative coupling provides a ready solution.

Nominally, APSIS will be designed to be compatible with the EXPRESS Pallet System on International Space Station. However, the choice of common interface standards (e.g. RS422 for Data and +28V DC for power) wil1 enable any other flight oppo~unities to be exploited, On the ISS another aerogel-based ‘hybrid’ dust detector and collector system to measure impact flash has been proposed to the JEM Exposed Facility as well (Yauo and Kitazawa, 1998).

Cleanliness might be thought to be a significant factor in residue analysis, but in impactor analysis, the ambient cleanliness levels may be fairly “rough” compared with the demands of optical experiments, due to the compaction and concentration effect of residues during impact. In contrast to a capture cell (McDonnell et al 1984), where residue is vapourised and

lmpactor vector

w44, 2

Chemistry { 11

Diameter (2)

1‘ / x, y. z position

Fig. 6. The particulate info~ation base available for each particle comprises detection position and velocity vector V(0, 9) together with the estimated diameter and residue chemistry. Ready distinction between space debris and meteoroids is made; for natural particulates the sources can be delineated and identified as the database is developed.

APSIS - Instrument Impact 321

spread over a larger area, intact residues when examined with a microprobe may effectively fill the sensitive detection volume to 100%. Capture within aerogel thus offers a good tolerance to general contamination.

In terms of collector area, aerogel also offers unique properties. The prime cost factor in space ~pioy~nt is, in fact, mass ! Aerogel itself is thus, in principle, almost free to launch, even in the large areas needed for Space Station; in reality, costs are also determined by the detector interfaces and experiment housing. However, the principle of the economy of scale applies here to large detectors and, since there is threshold mass for any experiment, parametric analysis shows that the ~~g~~u~ mass (and cost) for increasing the detector volume are extremely low because of the threshold mass of the support structure for a small experiment. The relative mass (and cost) per unit collection area decreases linearly with the detector dimensions up to the limit of an acceptable maximum limit for an exposure unit.

CONCLUSIONS

The use of existing, and space proven, particle detector techniques provides adequate design data for a new dimension in cosmic dust and space debris research. It is the cumbi~tion o~techniq~es which breaks new ground and can provide for each detected particle, within the instrument performance bands, the opportunity for unique determination of its origin.

For the new International Space Station _ where areas of detector may extend to the interception of not only large numbers of particles, but also large masses - APSIS, indeed, is ready for development as a system.

The principal author acknowledges PPARC UK for support; and Margaret Fowler and Mary Watts for the usual typographical demands. Hajime Yano holds a National Research Council (NASA/JSC) Research Associateship.

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